S8 and S21

Rent Arrears

Hi, My tenants (a family) owes more than half of October rent and I already know they can't pay November rent due on 8th November.
I said for the last time, I'll wait for them to pay this by end of November. (I kept giving them more time to pay in good faith since August, and they kept promising me to get the payment up to date.)

I want them to leave,and I know I can't use Section 8 letter as this is short hold tenancy and the first 6 months has lapsed. I have Section 21B I served back in April08, and I haven't renewed the contract after 6 months, so this S21B is still valid as today, but can I use this to gain possession for tenants with rent arrears?
I made 2 copies and signed both, gave one to them and I have one copy. The tenants haven't signed them as there were no place for them to sign. Will this be a problem?

Please tell me what is the best action to take.

Thank you.